Dusts of heavy metals fall to Earth's surface as the solar system drifts through pockets of the material, which were produced by supernovas more than 10 million years ago. An analysis of those metals suggest that other extreme celestial events also produced heavy elements, possibly changing the story of the solar system's origins.
An international team of researchers analyzed radioactive isotopes of iron and plutonium and found plutonium was being added to the crust too slowly for supernovas to be the metals' only source. In a Science paper that will be published Friday, the authors suggest that the plutonium may be from mergers of neutron stars or other explosive events, which may predate the solar system.
"It's the question: Where and how are the elements produced in nature — what's going on up there in the sky?" said Anton Wallner, the paper's lead and senior author and a nuclear physics research fellow at the Australian National University. "One major open question in physics is where and how are the heavy elements produced."
At the core of the sun and most other stars, hydrogen undergoes nuclear fusion to create helium, and the two light elements constitute 98% of visible matter in the universe. While hydrogen, helium and some lithium were created soon after the Big Bang, almost all other elements were formed late in the life cycle of stars.
About half of these heavier elements are created when stars swell into giants and fuse non-hydrogen elements. The other half are born in more violent conditions such as supernovas, the dying explosions of massive stars. In the latter scenario, during the rapid-neutron capture process, or r-process, atoms quickly accumulate neutrons and some of those neutrons decay into protons, creating heavier elements.
The "stardust" that dead stars leave behind sometimes forms into new stars, planets and other features of stellar systems.
Wallner and his co-authors investigated heavy metals from more recent supernovas that occurred in the sun's neighborhood in the last 12 million years. The solar system drifts through clouds of their remnant matter, and some of it falls to the Earth's surface.
The researchers targeted iron-60 and plutonium-244, which are isotopes — forms of an element with a different number of neutrons in their nuclei — that only form in supernova-like conditions.
The researchers sampled a slice of oceanic crust from about 1,500 meters (4,900 feet) below sea level in the Pacific Ocean, allowing them to test isotopes for the last 10 million years.
After counting the atoms of each isotope in the sample and subtracting the plutonium-244 expected to be from tests of nuclear weapons, the scientists found that the concentrations of the metals peaked twice at roughly the same times, which possibly correspond to individual supernovas.
The ratio between the rates at which plutonium and iron were added to Earth's crust was nearly constant throughout the studied time period, suggesting that the arrival patterns of the metals were similar. The ratio, however, was too low for both to be produced by supernovas alone — there was less plutonium being added than expected. A previously unconsidered source of heavy elements may have been involved, according to the study's authors.
They speculated that supernovas may not be the dominant producers of heavy elements, as previously believed. While the iron-60 is probably created by supernovas, the team said, the plutonium-244 may have been created by mergers of neutron stars, ultradense city-sized objects left behind by some supernovas. These mergers are so extreme that they can create gravitational waves that ripple through spacetime, and a 2017 detection of a neutron star merger showed such events can also make heavy elements through the r-process.
A neutron star merger or another extreme event may have occurred before the solar system was created, and added large amounts of heavy elements into space, which would then be swept up with the iron-60 following a supernova and make their arrival on Earth appear simultaneous.
"The presence of interstellar 244Pu can originate from the same supernovae that produced the 60Fe," Wallner said. "It could, however, also be the remaining small fraction of an earlier rare event, that at that time would have produced a large amount of r-process nuclides including 244Pu."
Determining the origins of these two isotopes would inform scientists' understanding of how the solar system and other star systems formed. If neutron star mergers or other rare events created some of the plutonium-244 detected in this study, our solar system "very likely" formed from some of their products as well, according to Wallner. Earlier research has suggested there may have been a neutron star merger about 80 million years before the solar system was created.
"We learn about the recent history of our galactic neighborhood," Wallner said. "This is to understand what makes half of the heavier elements in nature."
Wallner said he and his colleagues are currently analyzing a sample 10 times the size of the one in the new paper, which should clarify when the spikes in the two isotopes occurred and whether events other than supernovas may have created them. The team will also begin studying other radioisotopes that are expected to have extraterrestrial sources.
The study, "60Fe and 244Pu deposited on Earth constrain the r-process yields of recent nearby supernovae," published May 14 in Science, was authored by A. Wallner, Australian National University and Helmholtz-Zentrum Dresden-Rossendorf; M. Froehlich, S. Pavetich and S. Tims, Australian National University; M. Hotchkis, Australian Nuclear Science and Technology Organisation; N. Kinoshita, Shimizu Institute of Technology; M. Paul, The Hebrew University of Jerusalem; M. Martschini, University of Vienna; N. Kivel and D. Schumann, Paul Scherrer Institute; M. Honda, Japan Atomic Energy Agency; and H. Matsuzaki and T. Yamagata, University of Tokyo.